This invention relates generally to methods of doping semiconductor devices and, more particularly, to methods for increasing the electrical activation of dopants in semiconductor (e.g., silicon) devices.
Integrated circuits (ICs) commonly include semiconductor devices such as field effect transistors (FETs) or bipolar junction transistors (BJTs), which have junctions formed by introducing impurities (i.e., dopants) into a semiconductor body (e.g., a substrate or an epitaxial layer). The dopants are used to form particular regions of the device (e.g. the source and the drain of an FET or the emitter of a BJT).
Ion implantation is a commonly used method for introducing dopants in a controllable, reproducible fashion that is free of most undesirable side effects. During ion implantation, dopant atoms are ionized, accelerated, and directed at the semiconductor body. Illustratively the body is a single crystal silicon (Si) substrate. However, other substrate materials, such as Group. III-V compounds, are also suitable, but for simplicity we continue this exposition assuming the substrate material is Si. The dopant atoms enter the crystal lattice of the substrate, collide with Si atoms, gradually lose energy, and finally come to rest at some depth within the crystal lattice. The average depth is controlled by adjusting the acceleration energy. The dopant dose is controlled by monitoring the ion current during implantation.
One consequence of ion implantation is that defects, produced when dopant ions collide with substrate atoms, are introduced into the crystal lattice. Post-implant thermal treatments of the implanted wafer are commonly required to repair, or anneal, the lattice damage. These treatments also place dopants ions on substitutional sites within the Si substrate where the expectation is that they will be electrically active. However, because the implantation process often introduces a concentration of dopant atoms that is higher than the solubility limit, dopant atoms can precipitate out of the lattice and become electrically inactive. In addition, even below the solubility limit the damage from the implantation process can lead to segregation and/or clustering of dopant atoms. These clusters or agglomerates are immobile and electrically inactive.
A model that explains clustering and deactivation for the case of doping by boron (B) atoms in single crystal Si substrates has been presented by L. Pelaz et al., Appl. Phys. Lett., Vol. 70, No. 17, pp. 2285-2287 (1997), which is incorporated herein by reference. According to this model, during implantation and prior to post-implant annealing, implanted B atoms form B-interstitial complexes in the presence of a high-concentration of Si interstitials. During annealing these complexes evolve into higher-order clusters containing several B atoms per Si interstitial. The B atoms contained in these clusters are immobile and electrically inactive. According to our interpretation of this model, if the interstitials from the B implant were removed, the formation and evolution of B interstitial complexes would be suppressed, thereby increasing the electrical activation of the implanted B profile.
Recently, V. C. Venezia, et al., Appl. Phys. Lett., Vol. 74, No. 9, pp. 1299-1301 (1999), have confirmed that implanting high-energy Si ions into Si substrates creates interstitial-defects near the projected ion range of the implant (Rp) and vacancy defects between the surface and Rp. They also showed that the vacancy defects created by a 2 MeV Si implant annihilated the interstitials created by a lower energy 40 keV Si implant, thereby eliminating B-enhanced diffusion and interstitial cluster formation. Accordingly, it would appear at first blush that the elimination of interstitials from a dopant implant by a high-energy Si co-implant should increase the electrical activation of the B implant after post-implant annealing. However, two reports, one by Larsen, et al., Nuc. Inst. Meth. Phys. Res. B, Vol. 112, pp. 139-143 (1996) and the other by S. Saito et al., Appl. Phys. Lett., Vol. 63, No. 2, pp. 197-199 (1993), demonstrate that the desired increase in activation has not been realized. Both of these articles are incorporated herein by reference. Larsen et al. co-implanted higher energy Si ions into pre-amorphized Si substrates prior to a lower energy dopant implant (BF2) and observed an actual reduction in the electrical activation of the dopant for rapid thermal annealing (RTA) temperatures below about 850xc2x0 C. Likewise, Saito et al. co-implanted high energy Si or As ions into pre-amorphized Si substrates after a lower energy dopant implant (B) and observed essentially no change in the electrical activation of the dopant for RTA temperatures of about 1000-1100xc2x0 C.
Thus, a need remains in the art for an effective technique for increasing the electrical activation of ion-implanted dopants in semiconductor bodies in general and in Si substrates in particular.
In addition, there is a need for such a technique that does not require any pre-amorphizing steps.
There is also a need for such a technique that entails only low temperature process steps (i.e., less than about 800xc2x0 C. for Si) following the completion of ion implantation.
We have found that under certain prescribed conditions a co-implantation process can be effective in increasing the electrical activation of implanted dopant ions. In accordance with one aspect of our invention, a method of making a semiconductor device includes the steps of providing a single crystal semiconductor body, implanting vacancy-generating ions into a preselected region of the body, implanting dopant ions into the preselected region, the dopant implant forming interstitial defects in the body, and annealing the body to electrically activate the dopant ions. Importantly, in our method the vacancy-generating implant introduces vacancy defects into the preselected region that are effective to annihilate the interstitial defects. In addition, process steps that amorphize the surface of the implanted region are avoided, and the dose of the vacancy-generating implant is made to be greater than that of the dopant implant. In a preferred embodiment, the peak of the vacancy defect concentration profile substantially overlaps the peak of the dopant implant concentration profile. In another preferred embodiment the peak of the vacancy-generating implant profile is deeper than that of the dopant profile.
In accordance with another aspect of our invention, after ion implantation is complete only low temperature process steps (e.g., steps performed at temperatures no greater than about 800xc2x0 C. for Si devices) are performed.